We know that gravity pulls a bullet downward whether shooting level (parallel to the earth), uphill, or downhill. If your scope is set properly, and you shoot either uphill or downhill, your bullet will hit high, unless you compensate for the angle.
You can buy an angle indicator for as little as $20 on eBay. Some mount on the scope, some on the rail, and some even can mount to the rail underneath the scope. If your scope takes your whole rail, you will likely have to add a hundred dollars or more to the cost to get one that will mount correctly.
Knowing the correct angle is important. Guessing is not a good idea. The Cosine of the angle will give you your True Ballistic Distance (TBD). This is the number that is actually important. The Cosine means nothing if you don’t calculate your true ballistic distance.
If you are shooting level, lets say 500 yards, then your target is straight ahead 500 yards. Duh! But when you shoot at an angle, the distance to the target horizontally is different than the distance the bullet travels. Lets say you are shooting downwards at a 45 degree angle at a target which as the bullet travels is 500 yards. The horizontal distance between you and the target is less.
To determine the true ballistic distance or the distance you need to have your sights set for in order to hit your target, you simply read the angle from your angle indicator and use the corresponding cosine. The cosine will be a number that is less than one. In other words the cosine will be a decimal number. You multiply the distance the bullet travels (500 yards) by the cosine, which reading the table below indicates that a 45 degree angle up or down has a cosine of .71. So you multiply 500 yards by .71 and get 355 yards, which is the true ballistic distance. When you aim for 355 yards instead of 500, your bullet will release brain matter from your intended target.
It’s actually quite simple. When you are shooting at an angle, either up or down, gravity has less effect on your bullet than when you are shooting horizontally. In the example above, you would have overshot your target considerably if you hadn’t adjusted for the true ballistic distance.
Depending on your rifle, a bullet may drop 20 or more inches by the time it travels 500 yards. And if your scope is set to be accurate at 500 yards, your TBD is only 355 yards which in that distance the bullet may only drop 7 inches or so on a .30-06 so without calculating for angle you would have overshot your target by more than a foot.
And, in case you are wondering, shooting straight down, as in a 90 degree angle, gravity will pull the bullet straight towards the target so the cosine is zero. So when you multiply your bullet distance by zero, you get zero, and therefore the trajectory will be the same as if the target was zero inches from the end of your barrel.
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